The present invention relates to knife gate valves, and in particular, in such a valve, a resilient seat that has a semi-hard, self-lubricating sealing surface which contacts the blade portion of the gate as it is lowered to close the valve.
Knife gate valves have traditionally been used to control the flow of heavy slurries transported through tubular conduits. The gate has a blade portion with a beveled edge to cut through the material suspended in the slurry as the gate is lowered to seal off the flow through the valve. The seat is typically the metal valve body, and the material in the slurry assists in sealing the valve.
A problem associated with using a knife gate to control the flow of slurries of light viscosities is that the material accumulating on the surface of the valve seat does not promote a tight seal. Knife gate valves controlling the flow of light viscosity slurries typically employ, therefore, a seal that is made of a resilient substance. The material accumulating on the seat tends, however, to cause the gate to adhere to and pull out the seat when the gate is retracted to open the valve. This is especially true for valves that control the flow of nonlubricating fluids. Lubricating the gate contacting surface of the valve seat can reduce surface friction and thereby prevent adhesion of the gate to the seat. Lubricating the surface of the seal of a knife gate valve precludes, however, its use in conduits which transport certain substances, such as, for example, air and natural gas, which would be contaminated by a lubricating substance.